U.S. patent application number 11/872693 was filed with the patent office on 2008-04-10 for porous silicon composite structure as large filtration array.
Invention is credited to Timothy Joseph Dalton, Michelle Leigh Steen.
Application Number | 20080083697 11/872693 |
Document ID | / |
Family ID | 34550270 |
Filed Date | 2008-04-10 |
United States Patent
Application |
20080083697 |
Kind Code |
A1 |
Dalton; Timothy Joseph ; et
al. |
April 10, 2008 |
POROUS SILICON COMPOSITE STRUCTURE AS LARGE FILTRATION ARRAY
Abstract
A novel asymmetric filter membrane, and process for making is
disclosed in several exemplary versions. The membrane structure is
physically robust and suitable for use in a wide variety of
applications. The support membrane is may be comprised of material
such as a porous silicon or a silicon oxide, and the separation
membrane may be comprised of material such as a polymer, zeolite
film, or silicon oxide. The process relies on steps adapted from
the microelectronics industry.
Inventors: |
Dalton; Timothy Joseph;
(Ridgefield, CT) ; Steen; Michelle Leigh;
(Danbury, CT) |
Correspondence
Address: |
DAVID AKER
23 SOUTHERN ROAD
HARTSDALE
NY
10530
US
|
Family ID: |
34550270 |
Appl. No.: |
11/872693 |
Filed: |
October 15, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10697077 |
Oct 30, 2003 |
7282148 |
|
|
11872693 |
Oct 15, 2007 |
|
|
|
Current U.S.
Class: |
216/2 |
Current CPC
Class: |
B01D 69/12 20130101;
B01D 71/02 20130101; B01D 2325/022 20130101; B01D 2325/08 20130101;
B01D 69/10 20130101; B01D 71/64 20130101; B01D 67/0079 20130101;
B01D 67/0034 20130101; B01D 67/0062 20130101; B01D 67/0072
20130101; B01D 2325/24 20130101; B01D 71/68 20130101 |
Class at
Publication: |
216/002 |
International
Class: |
C23F 1/00 20060101
C23F001/00 |
Claims
1. A process for fabricating a composite microfilter structure
comprising: etching a pattern of micropores through a standard-size
semiconductor wafer to form a support membrane; and providing at
least one separation membrane atop the support membrane.
2. The process recited in claim 1, wherein the step of providing a
separation membrane comprises providing an organic separation
membrane.
3. The process recited in claim 3, wherein the organic separation
membrane provided comprises a thermoplastic polymer.
4. The process recited in claim 2, wherein the organic separation
membrane provided comprises a polymer that will cleanly and
efficiently transmit a permeate to a support layer.
5. The process recited in claim 3, wherein the thermoplastic
polymer is selected from the group consisting of polyimide, SiLK,
polysulfone, and polyethersulfone.
6. The process recited in claim 1, wherein the step of providing a
separation membrane comprises providing an inorganic separation
membrane.
7. The process recited in claim 6, wherein the step of providing an
inorganic separation membrane comprises providing an inorganic
membrane that will cleanly and efficiently transmit a permeate to a
support layer.
8. The process recited in claim 6, wherein the inorganic separation
membrane is selected from the group consisting of silicon, silicon
dioxide, zeolite and any combination thereof.
9. The process recited in claim 6, wherein the step of etching a
pattern of micropores in a semiconductor wafer comprises dry
etching a silicon-containing wafer by fluorine radicals in a plasma
using TMDE.
10. The process recited in claim 6, wherein the step of providing
an inorganic separation membrane comprises depositing an inorganic
material by a method selected from the group consisting of CVD and
plasma-enhanced CVD.
11. A process for fabricating a silicon oxide membrane for a
composite asymmetric microfilter structure, comprising: depositing
a silicon oxide atop a support membrane by a method selected from
the group consisting of CVD and plasma-enhanced CVD with a TEOS
source; depositing and curing a photoresist layer atop the silicon
oxide; providing the photoresist layer with a pattern for a
plurality of micropores; exposing and developing the pattern in the
photoresist layer; transferring the pattern into the silicon oxide
using a dry etch method; and removing the remaining photoresist.
Description
[0001] This application is a divisional of application Ser. No.
10/697,077, filed on Oct. 30, 2003, which is incorporated herein in
its entirety.
BACKGROUND
[0002] Microfilters are used in separation processes in which
solids, liquids or gasses are separated from fluid media. The
separation can be on a molecular level or on a coarser level. The
present invention relates to the use of a composite microfilter
comprising a separation layer and a porous support layer. This type
of configuration is known as asymmetric. Aspects of the present
invention are conveniently adapted to the art of filtration from
processes well established in the art of semiconductor
processing.
[0003] A simple composite filter typically comprises a thin
polymeric membrane separation layer undergirded by a support layer.
The separation layer may include very small diameter pores or it
may be dense, i.e. essentially opening-free, as in osmotic
filtration by absorption and diffusion through a semipermeable
membrane. The separation layer may be on the order of about 1-2
microns thick. A thin polymeric layer tends to favor increased
permeability, but also tends to promote frailty of the filter. The
support layer is often also a membrane, of the same or different
composition as comprises the separation membrane, having a matrix
of pores of larger diameter than those in the separation membrane,
the pores extending therethrough to the supported separation
membrane. A support layer is generally about 12 to about 125
microns thick, but is always much thicker than the separation
layer.
[0004] There may be more than two membrane layers in a particular
filter structure and as many types of material, as described in
U.S. Pat. No. 6,596,112 B1 to Ditter et al., in which melt bonding
of stacked materials, at a temperature as low as under 200.degree.
F. to above 396.degree. F. is used to build a laminated multilayer
structure. There also may be additional components within a filter
chamber, such as that described in U.S. Pat. No. 6,302,932 B1 to
Unger et al, in which a rigid frame of chemical and
moisture-resistant metal or polymer can be added to reinforce the
support of the separation layer.
[0005] As a fluid medium passes through the separation chamber, one
or more of the materials within the fluid are separated by
molecular or particle size or by chemical affinity for the
separation layer, while the porous support membrane, having larger
diameter pores than in the separation layer, offers little or no
resistance to mass transfer and adds to the mechanical strength of
the much thinner separation layer. Pore size distribution in the
separation layer promotes selectivity in a filtration system which
does not rely on osmosis. Pore size in an asymmetric membrane can
be fabricated to be as small as about 0.01 micron, or even less,
and as great as about 100 microns, or even greater, as required by
the materials to be separated. Pore distribution may comprise as
little as about 3% of the membrane or as much as about 80%.
[0006] Asymmetric membranes have been successful as commercial
membrane filters, as they offer high selectivity through the
separation layer and a high throughput of material through the
supporting layer. Asymmetric membranes are found in a number of
important fluid-fluid, gas-gas, solid-fluid, solid-gas and
gas-fluid filtration processes, which may include ink filtration,
semiconductor processing, air and water purification, water
desalinization, environmental sampling, gas recovery from
manufacturing processes, enrichment of specific gases in a mixture,
pharmaceutical manufacture and manufacture of high purity
chemicals, purification and testing in the food and beverage
industries, blood filtration diagnostics and dialysis in the
medical field, quick sample detection, and thin film
chromatography.
[0007] The selection of materials for inclusion in the filter is
highly solute- and solvent-dependent. Although commercial
asymmetric membranes are generally fabricated from thermoplastics,
such as polysulfone, because they are thermally robust and somewhat
chemically resistant when compared to other polymers, a membrane
need not be comprised solely of an organic material. Under
conditions unfavorable to polymer use, it may be comprised of a
mineral such as the carbon-containing or ceramic membranes
described in U.S. Pat. No. 5,190,654 to Bauer. A membrane may
comprise a porous metal layer, such as the metal coated with
sintered particles described in U.S. Pat. No. 5,492,623 to Ishibe
for use at temperatures up to about 400.degree. C. in the
filtration of process gas used in manufacturing semiconductors.
U.S. Pat. No. 6,605,140 B2 to Guiver et al. refers to a
polyimide-silica membrane.
[0008] While the present invention is configured in essentially
smooth layers, a membrane may be more highly dimensional,
fabricated into shapes in steps modeled on thin film etching,
lithographic patterning and deposition practiced in the production
of microelectronics, as described in U.S. Pat. No. 4,701,366 to
Deckman et al. As described in the Deckman et al. patent,
zeolite-like materials of controlled pore size between about 10 and
about 10,000 Angstroms, but possibly as small as about 5 Angstroms,
are fabricated as slots in shapes etched in a superlattice
structure situated on a potentially removable substrate. In
addition to zeolite, layers that are described as candidates for
the superlattice structure include silicon nitride, amorphous
silicon, amorphous germanium and amorphous silicon dioxide,
deposited sequentially by evaporation and sputter deposition.
[0009] Permeability of a membrane may be improved by applying
positive pressure on the separation side and/or by negative
pressure on the support side. One approach to improving permeance
of a porous alumina ceramic membrane is described in U.S. Pat. No.
5,782,959 to Yang et al. in which the alumina pores are provided
with a catalytic palladium coating in order to facilitate hydrogen
separation from gas streams. In another type of support membrane,
the Immobilized Liquid Membrane, described in U.S. Pat. No.
5,100,555 to Matson, permeability is influenced by the depositing
into the pores of the membrane by capillary action any of a number
of identified solvents, some of which have been approved by the
United States Food and Drug Administration for use in food and
medical applications.
[0010] A membrane may be or be rendered, hydrophobic, hydrophilic
or oleophobic, depending on the intended application and the nature
of the materials to be separated. Commercial organic asymmetric
membranes are generally constructed from thermoplastics. However,
problems associated with the natural hydrophobicity of
thermoplastics severely limit the use of these materials in many
water-based applications. As a consequence, the surface, and in
some cases the interior, of these membranes must be rendered
hydrophilic through the addition of a wetting agent, such as a
dilute detergent solution, and/or by chemical modification of the
membrane structure prior to use in aqueous separation. Issues
frequently encountered in applying these modifications include lack
of permanence of the modification, fouling of the filtered material
by the impermanent wetting agent or chemical, reduction in porosity
of the membrane, and the presence of unmodified areas in the porous
structure.
[0011] Some materials used to modify the surface charge of a
membrane include polyvinylpyrrolidone, polyethylene glycol,
polyvinyl alcohol, carboxymethyl cellulose or mixtures thereof. As
described in U.S. Pat. No. 5,032,149 to Hayes, some materials
applied on particular membrane surfaces to improve selectivity
either in gas-gas or liquid-liquid separations may include a
fluorinated amphophilic compound, a Bronsted-Lowry acid or base or
a dilute cationic, anionic or nonionic surfactant solution.
[0012] Other issues that plague long-term use of treated membranes
include increased risk of embrittlement, shrinkage, and pinhole
defects. The membrane industry incurs an inordinate expense in
developing alternate membrane materials and broadening the types of
membrane surface chemistries. In one example, U.S. Pat. No.
6,110,249 to Medcalf et al., is described a microporous
e-polytetrafluoroethylene membrane for removing particles from gas,
the membrane and a support layer being melt bonded in order to
reduce pore blockage between the layers, thereby reducing the
incidence of tearing and cracking attributed to partial pore
nonalignment. Another approach includes sealing defects with a
swelling agent or applying a thin overcoat of dense polymer. In
U.S. Pat. No. 4,775,474 to Chau et al. a glassy polymer, normally
dense, is strengthened by crazing on its major surface intended for
contact with the permeate stream subsequent to controlled
crosslinking to a limited surface depth.
[0013] Solutions to membrane problems may introduce limitations of
their own, such as altering the permeation and selectivity,
incomplete coverage or delamination of the plug or overcoat with
use, and heat degradation. Notably, silicon, a membrane material in
the present invention, is naturally and permanently hydrophilic.
Silicon is not adversely affected by drying, and its natural
hydrophilicity does not elute.
[0014] The art, both prior and current, describes a number of
complex multistep processes for fabrication of filters for
particular separations and in particular applications. U.S. Pat.
No. 6,565,782 B1 and U.S. Pat. No. 6,045,899 to Wang et al. review
in the art the formation of asymmetric, hydrophilic microfiltration
membranes fabricated using a typical sol-gel phase inversion
process, involving a number of steps to obtain the gel, which is
the polymer matrix. The Wang patents also review the fabrication of
polymeric membranes by casting from homogenous solutions of
polymer, citing that the resulting membranes are not usually as
asymmetric as those cast from an inversion process and may even
have reverse asymmetry. U.S. Pat. No. 6,486,240 B1 to Won et al.
describes issues involved in the fabrication of membranes by
gelation, and describes an alternative phase separation gelation.
Another set of processes for fabricating polymeric asymmetric
membranes, for separation of certain liquids and of certain gases,
is found in U.S. Pat. No. 4,873,037 to Chau et al. The Chau et al.
patent describes several modes of fabrication and is also useful
for its description of other fabrication methods for asymmetric
membranes in the art, as well as listing a number of uses.
[0015] Despite the work reported in the field of membrane
development, the need remains for the unique combination of
materials and processes set forth in the several embodiments of the
present invention, in which robust versatile membranes are provided
relatively simply and cleanly, using steps adapted from the
microelectronics art.
SUMMARY OF THE INVENTION
[0016] The present invention is directed to a membrane microfilter
having a thin separation layer and a porous silicon substructure,
and methods for making the same. The invention is set forth in
several embodiments. The present invention offers several important
advantages over commercial polymeric membranes. Silicon offers a
wide range of materials and surface chemistries for compatibility
with a wide range of filtration systems. The 1410.degree. C.
melting point of silicon implies that it is a refractory material
suitable for higher temperature filtrations; it can also withstand
low temperatures. Silicon is relatively inert to chemical attack,
except by halogens, alkali solutions, HF and HNO.sub.3. Silicon
wafers are easy to handle and do not easily tear, crack or suffer
other insults during normal handling and use, which significantly
reduces or eliminates down time due to repair and reduces the
opportunity to introduce fouling during repair. Silicon wafers are
not expensive, and are commercially available in 5 inch, 6 inch, 8
inch and 12 inch diameter sizes which can be combined to create
large filtration arrays for optimal throughput. Silicon is a
suitable support membrane for a polymer that is, or can be modified
to be, positively photoactive, or a polymer or inorganic that can
be dry etched through a mask.
[0017] A support membrane comprised of other semiconductors such as
silicon dioxide, silicon nitride or germanium shares many of the
advantages of silicon, including the advantage of handling
experience in the microelectronics industry.
[0018] The number of steps in the fabrication of the membrane of
the present invention are minimal and are less complex than those
known in the art for fabricating membranes, such as phase inversion
and phase separation processes. The process of the invention relies
on steps commonly practiced using the extant semiconductor
processes, clean room facilities and semiconductor tooling used in
FEOL/BEOL (Front End of Line/Back End of Line) microelectronics
technologies, wherein obtaining features in the submicron range is
routine. Fouling is a major concern in the filter art, including
fouling that originates in the filter itself. With the simpler,
clean, processes and fewer, but more robust materials and process
steps of the present invention, cleanliness is favored as
fabricated and as cleaned for reuse. Alternatively, the filter is
cheap enough to discard after use. It can be implanted in the body.
The present invention, which incorporates a lithographic process,
can provide a higher packing density than polymer-based structures
fabricated by phase inversion or phase separation processes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic representation of a porous separation
layer disposed atop a porous silicon support membrane.
[0020] FIG. 2 is a schematic representation of a dense polymer
separation layer disposed atop a porous silicon support
membrane.
[0021] FIG. 3 is a schematic representation of a porous inorganic
separation layer disposed atop a porous silicon support
membrane.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The filter membrane structure shown in FIG. 1 uses a thin
separation layer 1, which can be an inorganic, for example
SiO.sub.2, or an organic material, that is lithographically
patterned by a method known in the art to form a channel-pore
structure. A channel-pore structure permits a size-selective
process in which molecules larger than the diameter of the
channel-pores are retained, while the smaller molecules elute. The
thin separation layer 1 is deposited on a bulk crystalline silicon
support membrane 2. Spin glass, such as siloxanes, silsesquioxanes,
N-silsesquioxanes, and polycabosilanes also can be used to form a
separation layer, as can polyimide, polysulfone, and
polyethersulfone.
[0023] The silicon material comprises a wafer thickness which, as
manufactured, is between about 725 and about 750 .mu.m not mm
thick. Thin film separation layer 1 can be deposited by a number of
methods known to those skilled in the art, such as chemical vapor
deposition (CVD), plasma-enhanced CVD, or spin-on. In an exemplary
embodiment, the separation layer comprises a silicon oxide formed
by a plasma-enhanced CVD process with a tetraethoxysilane (TEOS)
source in a process known in the art. Next, a photoresist layer is
deposited upon the oxide layer and cured, using conventional
photoresist processing techniques. The photoresist layer is then
patterned, preferably with an optical aligner and a photomask,
exposed and developed to create openings in the photoresist layer.
Then, using the resist layer as a masking layer, the pattern is
transferred into the underlying oxide by a dry etching method using
a LAM4520XL etch chamber and C.sub.4F.sub.8/CO/Ar/O.sub.2
chemistry. Then, the resist is stripped from the oxide layer using
conventional photoresist processing techniques, such as a solvent
strip or an O.sub.2 dry etch (ashing) method. Notably, the present
invention is not limited to vias or through-holes but includes
other shaped structures apparent to those skilled in the art such
as lines, squares, and octagons.
[0024] The backside of the wafer to be fabricated into a silicon
support membrane is lithographically patterned using a similar
method. A deep reactive ion etch is used to transfer the features
laterally-defined by the masking layer into the bulk substrate. A
suitable deep etch method is described in pending application for
patent, Ser. No. 10/639,989, now U.S. Pat. No. 7,060,624, which is
commonly assigned with the present invention, and is incorporated
herein by reference.
[0025] In the presently preferred embodiment, support membrane 2
comprises silicon, so that pattern transfer is accomplished using
silicon etching by fluorine radicals generated in a plasma, as is
known in the art. Such deep silicon structures can be accomplished
using commercially-available, deep reactive ion etch (RIE) systems
such as the A601E, available from Alcatel. The deep RIE dry etching
method uses time-multiplexed deep etching (TMDE), a variation of
sidewall passivation, wherein etching and deposition cycles are
performed sequentially. During the deposition step, sidewalls
formed within support membrane 2 are passivated by a polymer
deposited from a plasma formed from the deposition precursor.
During the subsequent etching cycle, both the polymer and the
silicon are preferentially etched from the bottom of the membrane
trench by ion bombardment. By switching between etching and
deposition cycles, deep anisotropic structures having vertical
sidewalls can be realized with very high etching rates in silicon
membranes. A buried or backside oxide or metal layer may be used as
a stopping layer for the deep Si etch.
[0026] The resulting structure shown in FIG. 1 can be used in
filtration applications in which macromolecules, such as proteins,
are separated from fluids, such as plasma, water, milk or the like,
based on size, by the porous oxide layer. To assure a high
selectivity and throughput, a preferred embodiment uses a very thin
oxide layer, less than about 1 .mu.m thick, a feature size
selective to the size of the permeate, and a high pattern density.
The exact pattern density, or loading, which can be established by
mask selection, is generally between about 0.5% and about 50%.
However, it is possible to increase the loading above 50%, with the
tradeoff of a decrease in etch rate. The average diameter of deep
vias in the underlying silicon support structure can be made larger
than those in the separation layer, so that as the silicon
substructure 2 acts as mechanical support for the oxide skin layer
1 it also offers little to no resistance to mass transfer.
[0027] The embodiments shown in FIGS. 2-3 use a similar method to
prepare the underlying silicon support for the separation layer.
However, in the embodiments shown in FIGS. 2-3, the separation
layers 3 and 4, respectively, need not be
lithograghically-patterned to produce a porous structure. The
structure shown in FIG. 2 uses a thin organic film, such as a
polymer, as separation layer 3 to separate molecules based on
chemical affinity or permeability, supported by porous silicon
support structure 2. In this embodiment, the thin film can be
deposited by any of a number of methods known to those skilled in
the art, such as chemical vapor deposition, plasma-enhanced
chemical vapor deposition, and spin-on. Hence, small molecules such
as N.sub.2 and O.sub.2 can be separated based on their respective
rates of permeation through the nonporous skin layer.
Alternatively, molecules that have a chemical affinity for the
particular organic thin film can adsorb and diffuse through the
separation layer. In a preferred embodiment, a material highly
permeable to certain organic molecules, trade named SilK.TM.
(Trademark of Dow Chemical Company), a crosslinked aromatic
thermoset which is highly permeable to short chain aliphatic
compounds and can separate organic contaminants from waste water,
is used. Spin glass, such as siloxanes, silsesquioxanes,
N-silsesquioxanes, and polycabosilanes also can be used to form a
separation layer, as can polyimide, polysulfone, and
polyethersulfone. To assure high throughput, the separation layer
should be very thin, i.e. under one micron.
[0028] The structure in FIG. 3 uses for the separation layer 4 a
thin layer, about 1 micron, of molecular-cage compounds known as
zeolites to separate small molecules based on size. The thin film
can be deposited by spin-on. Small molecules are trapped within the
molecular-cage structure, permitting larger molecules to pass. If
necessary, pretreatment of the surface underlying the zeolite layer
can be used to improve adhesion.
[0029] Although the figures show only one membrane structure, an
indefinite number of individual membrane structures may be
fabricated simultaneously across a standard diameter silicon wafer
in fabricating a large filtration array microfilter for optimal
throughput. The membrane structure can also be used as a prefilter,
or in a chain of filters, each unit of the chain providing an
increased level of purity.
* * * * *